A significant number of surgical procedures involve modifying tissue in a patient's body, such as by removing, cutting, shaving, abrading, shrinking, ablating or otherwise modifying tissue. Minimally invasive (or “less invasive”) surgical procedures often involve modifying tissue through one or more small incisions or percutaneous access, and thus may be more technically challenging procedures. Some of the challenges of minimally invasive tissue modification procedures include working in a smaller operating field, working with smaller devices, and trying to operate with reduced or even no direct visualization of the tissue (or tissues) being modified. For example, using arthroscopic surgical techniques for repairing joints such as the knee or the shoulder, it may be quite challenging to modify certain tissues to achieve a desired result, due to the required small size of arthroscopic instruments, the confined surgical space of the joint, lack of direct visualization of the surgical space, and the like. It may be particularly challenging in some surgical procedures, for example, to cut or contour bone or ligamentous tissue with currently available minimally invasive tools and techniques. For example, trying to shave a thin slice of bone off a curved bony surface, using a small-diameter tool in a confined space with little or no ability to see the surface being cut, as may be required in some procedures, may be incredibly challenging or even impossible using currently available devices.
One area of surgery which would likely benefit from the development of less invasive techniques is the treatment of spinal stenosis. Spinal stenosis occurs when nerve tissue and/or the blood vessels supplying nerve tissue in the spine become impinged by one or more structures pressing against them, causing symptoms. The most common form of spinal stenosis occurs in the lower (or lumbar) spine and can cause severe pain, numbness and/or loss of function in the lower back and/or one or both lower limb.
FIG. 1 is a top view of a vertebra with the cauda equina (the bundle of nerves that extends from the base of the spinal cord) shown in cross section and two nerve roots branching from the cauda equina to exit the central spinal canal and extend through intervertebral foramina on either side of the vertebra. Spinal stenosis can occur when the spinal cord, cauda equina and/or nerve root(s) are impinged by one or more tissues in the spine, such as buckled or thickened ligamentum flavum, hypertrophied facet joint (shown as superior articular processes in FIG. 1), osteophytes (or “bone spurs”) on vertebrae, spondylolisthesis (sliding of one vertebra relative to an adjacent vertebra), facet joint synovial cysts, and/or collapse, bulging or herniation of an intervertebral disc. Impingement of neural and/or neurovascular tissue in the spine by one or more of these tissues may cause pain, numbness and/or loss of strength or mobility in one or both of a patient's lower limbs and/or of the patient's back.
In the United States, spinal stenosis occurs with an incidence of between 4% and 6% (or more) of adults aged 50 and older and is the most frequent reason cited for back surgery in patients aged 60 and older. Patients suffering from spinal stenosis are typically first treated with conservative approaches such as exercise therapy, analgesics, anti-inflammatory medications, and epidural steroid injections. When these conservative treatment options fail and symptoms are severe, as is frequently the case, surgery may be required to remove impinging tissue and decompress the impinged nerve tissue.
Lumbar spinal stenosis surgery involves first making an incision in the back and stripping muscles and supporting structures away from the spine to expose the posterior aspect of the vertebral column. Thickened ligamentum flavum is then exposed by complete or partial removal of the bony arch (lamina) covering the back of the spinal canal (laminectomy or laminotomy). In addition, the surgery often includes partial or complete facetectomy (removal of all or part of one or more facet joints), to remove impinging ligamentum flavum or bone tissue. Spinal stenosis surgery is performed under general anesthesia, and patients are usually admitted to the hospital for five to seven days after surgery, with full recovery from surgery requiring between six weeks and three months. Many patients need extended therapy at a rehabilitation facility to regain enough mobility to live independently.
Removal of vertebral bone, as occurs in laminectomy and facetectomy, often leaves the affected area of the spine very unstable, leading to a need for an additional highly invasive fusion procedure that puts extra demands on the patient's vertebrae and limits the patient's ability to move. Unfortunately, a surgical spine fusion results in a loss of ability to move the fused section of the back, diminishing the patient's range of motion and causing stress on the discs and facet joints of adjacent vertebral segments. Such stress on adjacent vertebrae often leads to further dysfunction of the spine, back pain, lower leg weakness or pain, and/or other symptoms. Furthermore, using current surgical techniques, gaining sufficient access to the spine to perform a laminectomy, facetectomy and spinal fusion requires dissecting through a wide incision on the back and typically causes extensive muscle damage, leading to significant post-operative pain and lengthy rehabilitation. Thus, while laminectomy, facetectomy, and spinal fusion frequently improve symptoms of neural and neurovascular impingement in the short term, these procedures are highly invasive, diminish spinal function, drastically disrupt normal anatomy, and increase long-term morbidity above levels seen in untreated patients.
Therefore, it would be desirable to have less invasive methods and devices for modifying target tissue in a spine to help ameliorate or treat spinal stenosis, while inhibiting unwanted damage to non-target tissues. Ideally, such techniques and devices would reduce neural and/or neurovascular impingement without removing significant amounts of vertebral bone, joint, or other spinal support structures, thereby avoiding the need for spinal fusion and, ideally, reducing the long-term morbidity resulting from currently available surgical treatments. Furthermore, such techniques may also remove both ligament and bone during spinal stenosis decompression. Such techniques could substantially cut ligament rather than substantially tearing, ripping, or shredding it, and remove ligament without leaving a substantial amount of target ligament present after the procedure. It may also be advantageous to have minimally invasive or less invasive tissue modification devices capable of treating target tissues in parts of the body other than the spine. At least some of these objectives will be met by the present invention.